The invention relates to a borosilicate glass of high chemicals resistance and to its uses.
Fused glass/metal seals which are used in a chemically corrosive environment, for example in the construction of chemical installations or reactors, require glasses which have a very high resistance to both acidic and basic media. Moreover, the thermal expansion of sealing glasses of this type has to be matched to the chemically highly resistant metals or alloys which are used. In this context, it is desirable for the coefficient of linear thermal expansion to be close to or slightly below that of the metal which is to be sealed, so that during cooling of the fused seal, compressive stresses are built up in the glass, these stresses first ensuring a hermetic seal and secondly preventing tensile stresses from building up in the glass, which would promote the occurrence of stress crack corrosion. When using Fexe2x80x94Nixe2x80x94Co alloys, e.g. Vacon(copyright) 11, with a coefficient of thermal expansion xcex120/300 of 5.4xc3x9710xe2x88x926/K, or zirconium (xcex120/300=5.9xc3x9710xe2x88x926/K) or zirconium alloys, glasses with an expansion coefficient xcex120/300 of between  greater than 5 and 6.0xc3x9710xe2x88x926/K are required as sealing glasses for fused glass/metal seals.
A crucial parameter for characterizing the workability of a glass is the working point VA at which the viscosity of the glass is 104 dpas. It should be low, since even slight reductions in VA lead to a considerable fall in production costs, since the melting temperatures can be reduced. Furthermore, a VA which is as low as possible is also advantageous in the production of the fused glass/metal seal, since it is then possible to avoid overheating the parts which are to be fused together, since fusion can occur either at a lower temperature or within a shorter time. Finally, when using glasses with a relatively low VA it is possible to prevent the seal being adversely affected and, in the most extreme circumstances, leaking as a result of evaporation and recondensation of glass components. Furthermore, the working interval of a glass, i.e. the temperature difference between the working temperature VA and the softening point EW, the temperature at which the viscosity of the glass is 107.6 dpas, is also of significance. The temperature range within which a glass can be worked is also known as the xe2x80x9clengthxe2x80x9d of the glass.
Applications as primary packaging material for pharmaceuticals, such as ampoules or small bottles, also require glasses which have a very high chemical resistance with respect to acidic and basic media and, in particular, a very high hydrolytic stability. Furthermore, a low coefficient of thermal expansion is advantageous, since this ensures a good thermal stability.
Furthermore, the physical-chemical behavior of the glass during its further processing is of importance, since this has an influence on the properties of the end product and on its possible applications.
If a preform made from borosilicate glass which contains alkali metals, e.g. a tube, is processed further under hot conditions to form containers such as ampoules or small bottles, highly volatile alkali metal borates evaporate. The evaporation products condense in cooler regions, i.e. deposits are formed on the vessels, which have an adverse effect on their hydrolytic stability. Therefore, this phenomenon is disadvantageous in particular for applications of the glass in the pharmaceuticals sector, for example as primary packaging material for pharmaceuticals. The patent literature has already described glasses which have high chemicals resistances but are in need of improvement in particular with regard to their hydrolytic stability and/or have excessively high working points and/or do not have the desired expansion coefficients.
DE 42 30 607 C1 proposes chemically highly resistant borosilicate glasses which can be fused to tungsten. They have expansion coefficients xcex120/300 of at most 4.5xc3x9710xe2x88x926/K and, according to the examples, working points of xe2x89xa71210xc2x0 C.
The borosilicate glasses described in the publication DE 37 22 130 A1 also have a low expansion of at most 5.0xc3x9710xe2x88x926/K.
The glasses described in patent DE 44 30 710 C1 have a relatively high SiO2 content, namely  greater than 75% by weight and  greater than 83% by weight of SiO2+B2O3 in combination with an SiO2/B2 O3 ratio of  greater than 8, and little Al2O3, a composition which does make them highly chemically resistant but leads to disadvantageously high working points. These glasses, which in some cases have levels of ZrO2 (up to 3% by weight) and the ZrO2-containing borosilicate glasses described in DD 301 821 A7 likewise have low thermal expansions of at most 5.3xc3x9710xe2x88x926/K and 5.2xc3x9710xe2x88x926/K and, in particular on account of their ZrO2 contents, are highly resistant to lyes, but relatively susceptible to crystallization.
The glasses described in DE 198 42 942 A1 and DE 195 36 708 C1, have very high chemicals stabilities, being classified as belonging to hydrolytic, acid and lye class 1. However, the abovementioned drawbacks also apply to these glasses, on account of their high levels of ZrO2.
Moreover, in the glasses of the prior art, the problem of the evaporation of alkali metals described above during the hot further processing of preshaped glass bodies will continue to occur.
This problem is neither referred to nor solved in DE 33 10 846 A1, which describes BaO-free laboratory glasses.
It is an object of the invention to find a glass which satisfies high demands both with regard to the chemicals resistance, i.e. belongs to lye class 2 or better, to hydrolytic class 1 and to acid class 1, and on workability and has little evaporation of alkali metals.
This object is achieved by borosilicate glass having a composition, in percent by weight based on oxide content of:
and optionally at least one standard refining agent in an amount sufficient for refining.
The glass according to the invention has an SiO2 content of 70 to 77% by weight, preferably of 70.5 to 76.5% by weight of SiO2. Higher levels would increase the working point excessively and reduce the coefficient of thermal expansion too far. If the SiO2 content is reduced further, in particular the resistance to acids would deteriorate. An SiO2 content of  less than 75% by weight is particularly preferred.
The glass contains 6 to  less than 11.5% by weight, preferably 6.5- less than 11.5% by weight, particularly preferably at most 11% by weight of B2O3. B2O3 reduces the working temperature and the melting temperature while, at the same time, improving the hydrolytic stability. This is because B2O3 bonds the alkali metal ions which are present in the glass more securely into the glass structure. While lower contents would not reduce the melting point sufficiently far and would lead to an increase in the susceptibility to crystallization, higher contents would have an adverse effect on the acids resistance.
The glass according to the invention contains between 4 and 8.5% by weight, preferably up to 8% by weight, of Al2O3. Like B2O3, this component bonds the alkali metal ions more securely into the glass structure and has a positive effect on the resistance to crystallization. At lower contents, the susceptibility to crystallization would rise accordingly and, in particular with high B2O3 contents, there would be an increased evaporation of alkali metals. Excessively high levels would make their presence felt in terms of an increase in the working-and melting points.
For the glasses according to the invention, it is essential for the levels of the individual alkali metal oxides to be within the following limits:
The glasses contain 4-9.5% by weight, preferably 4.5-9% by weight of Na2O. They may contain up to 5% by weight of K2O and up to 2% by weight, preferably up to 1.5% by weight of Li2O. The sum of the alkali metal oxides is between 5 and 11% by weight, preferably between 5.5 and 10.5% by weight, particularly preferably between 7.5 and  less than 10.5% by weight. The alkali metal oxides reduce the working point of the glasses and are of crucial importance for setting the thermal expansion. Above the respective upper limits, the glasses would have excessively high coefficients of thermal expansion. Furthermore, excessively high levels of the components would have an adverse effect on the hydrolytic stability. Furthermore, for cost reasons, it is recommended to limit the use of K2O and Li2O to the maximum levels indicated. On the other hand, an insufficient level of alkali metal oxides would lead to glasses with a thermal expansion which is too low and would increase the working and melting points. With a view to making the glasses resistant to crystallization, it is preferable to use at least two types of alkali metal oxides. Even small amounts of Li2O and/or K2O in the range of a few tenths of % by weight allow diffusion of the components/assemblies involved in constructing the crystal phase toward the nucleus to be impeded and can therefore have a positive effect on the resistance to devitrification.
As further components, the glass may contain the divalent oxide MgO in an amount of 0-2% by weight, preferably 0-1% by weight, and CaO in an amount of 0-2.5% by weight, preferably 0-2% by weight, particularly 0- less than 2% weight. The sum of these two components is between 0 and 3% by weight, preferably between 0 and  less than 3% by weight. The two components vary the xe2x80x9clength of the glassxe2x80x9d, i.e. the temperature range within which the glass can be worked. The different strengths of network-modifying action of these components makes it possible, by exchanging these oxides for one another, to adapt the viscosity to the requirements of the particular production and working process. CaO and MgO reduce the working point and are securely bonded into the glass structure. Surprisingly, it has been found that limiting the levels of CaO to small amounts reduces the evaporation of highly volatile sodium and potassium borate compounds during hot-forming. This is of particular importance for Al2O3 contents, while at high Al2O3 contents it is possible to tolerate relatively high levels of CaO. CaO improves the resistance to acids. The latter statement also applies to the component ZnO, which may be present in the glass in an amount of up to 1% by weight. Furthermore, the glass may contain up to 1.5% by weight of SrO and up to 1.5% by weight of BaO, which increases the resistance to devitrification. The sum of these two components is between 0 and 2% by weight. The glass is preferably free of SrO and BaO. Particularly for use as primary packaging material for pharmaceuticals, it is advantageous if the glass is free of BaO.
Furthermore, the glass may contain coloring components, preferably Fe2O3, Cr2O3, CaO, in each case in amounts of up to 1% by weight, while the sum of these components should also not exceed 1% by weight. The glass may also contain up to 3% by weight of TiO2. This component is preferably used when, for special applications of the glass, damage to a fused glass/metal seal by UV radiation or the release of UV radiation is to be prevented.
The glass may contain up to  less than 0.5% by weight of ZrO2, resulting in an improvement in the resistance to lyes. The ZrO2 content is limited to this low maximum level, since higher levels would excessively increase the working point. Secondly, high levels of ZrO2 increase the risk of flaws in the glass, since it is possible that particles of the relatively insoluble ZrO2 raw material will remain unmelted and will pass into the product.
The glass may contain up to 1% by weight of CeO2. At low concentrations, CeO2 acts as a refining agent, and at higher concentrations it prevents the glass from being discolored by radioctive radiation. Therefore, seals produced using a CeO2-containing glass of this type still allows visual checks for any damage, such as cracks or corrosion to the conductor wire, even after exposure to radioactive radiation. Even higher concentrations of CeO2 make the glass more expensive and lead to an undesirable inherent brownish-yellow coloration. A CeO2 content of between 0 and 0.3% by weight is preferred for applications in which the ability to avoid discoloration caused by radioactive radiation is not essential.
The glass may contain up to 0.5% by weight of Fxe2x88x92. This reduces the viscosity of the melt, which accelerates refining.
In addition to the components CeO2 and fluorides, for example CaF2, which have already been mentioned, the glass can be refined using standard refining agents, such as chlorides, for example NaCl, and/or sulfates, for example Na2SO4 or BaSO4, which the finished glass contains in standard amounts, i.e. depending on the amount and type of refining agent used, in amounts of from 0.005 to 1% by weight. If As2O3, Sb2O3 and BaSO4 are not used, the glasses are, apart from inevitable impurities, free of As2O3, Sb2O3 and BaO, which is advantageous in particular for use as primary packaging material for pharmaceuticals.